The present invention relates to an image reading device. More specifically, the invention relates to an image reading device capable of storing in a memory reference data for shading correction which is hardly affected by a scratch or stain on a correction plate.
In image reading devices used in facsimile machines, digital copiers, etc., an analog image signal produced by an image sensor consisting of photoelectric conversion elements such as CCDs (charge-coupled devices) is converted into a digital image signal by an A/D converter. However, such image reading devices are accompanied by an undesirable phenomenon that an output image is deteriorated due to uneven illumination by a light source, characteristics of a lens system, non-uniformity in the sensitivity of the photoelectric conversion elements, and other factors. Therefore, shading correction required to maintain high quality output images.
The shading correction in an image reading device performed in the following manner. In order to produce an output to be used as correction reference data, a white document (i.e., a collection plate) is prepared. Placed at a predetermined position, the correction plate is read by an image sensor to produce an analog signal of one line. The analog signal is converted by an A/D converter into an digital signal, which is stored into a RAM. In subsequent reading of real documents, digital data stored in the RAM is read out so as to be used as the correction reference data for image data.
However, in conventional image reading devices, if a scratch or stain exists at, or foreign matter such as dust is attached to the reading position of the correction plate, desired correction data cannot be obtained, preventing accurate shading correction. FIG. 9 exemplifies such a situation. If pieces of dust 42 or the like are attached to a correction plate 41 at its reading positions, shading correction reference data Ds having lower level portions corresponding to the dust positions is stored. Even if smooth reading output data Tr is produced by reading a document having a certain density, peak levels N called "streak noise" appear in output data Da after the shading correction, resulting in an output image deterioration.